1. Field of the Invention
The present invention relates to a technology for storing the same data in a plurality of storage devices.
2. Description of the Related Art
The various data used in computer systems are generally recorded on storage devices such as disk device. Usually, over time, the data recorded on the storage device are likely to be used less frequently. As keeping such unused or less used data in the storage device is expensive, a backup of the data from the storage device needs to be taken on removable media and the storage device needs to be freed by deleting the data, pushing up the cost in terms of employing operators for performing these processes. Therefore, to make the system cost-effective, an autonomous system needs to be in place to take a backup of data and delete the data.
Therefore, hierarchical storage management employs a virtual disk device that archives data autonomously without consuming resources of the host and that presents to the host a transparent disk storage space obviating the need for removable media. The virtual disk device manages user data in previously set blocks (block lengths).
Japanese Patent Laid-Open Publication No. 2005-275674 discloses a technology whereby a backup can be taken efficiently by adjusting the data length transferred at a time from the storage device to the removable media such as a magnetic tape device.
However, in the conventional virtual disk device, which manages user data in previously set block lengths, a backup of data is taken in block lengths. Therefore, if a large volume of data is involved, overhead occurs in the tape disk, hampering efficient backup process.
In other words, if the volume of the user data is not very much and amounts to one block length, all the data can be written to the tape device in one go. However, it takes several times to write the entire data to the tape device if the volume of the user data is such that the data needs to be managed in several blocks. One file mark is written after every block of data is written to the tape. As the tape device cannot be accessed when the file mark is being written, there is a waiting period before the next block can be written to the tape device when the next block of data is moved to the write position.
Thus, the number of waiting periods due to the tape device overhead increases with the volume of the data, adversely affecting the performance. This is equally applicable when reading user data from the tape device. To improve performance, therefore, one can consider reducing the number of times file marks is written by enabling varying the number of blocks of data according to the volume of data. However, user data can vary vastly in size and a single block length and it is not possible to determine a single block length that will work for all the volumes of data.
In other words, an efficient backup process needs to be devised that works for any data volume, thus improving the performance of the virtual disk device.